feat(backend): expose GET /api/v1/sites/{site_id}/current pour l'issue #29

Ajoute la dernière mesure d'un site (SiteService.current), en réutilisant
la vérification d'existence déjà en place pour GET /sites/{site_id} :
SiteService gagne une dépendance ReadingRepository, sur le modèle de
composition déjà utilisé par StatsService/SensorService. Un site connu
sans lecture rend 200 avec les champs de mesure à null et
data_quality="critical" ; seul un site_id absent rend 404.

apps/backend/app/services/site.py

@@ -1,8 +1,16 @@
 from collections.abc import Sequence
+from dataclasses import dataclass
+from datetime import datetime
+from typing import Literal
 
 from app.models.energy import Site
+from app.repositories.reading import ReadingRepository
 from app.repositories.site import SiteRepository
 
+DataQuality = Literal["good", "partial", "degraded", "critical"]
+
+QUALITES_CONNUES: frozenset[str] = frozenset({"good", "partial", "degraded", "critical"})
+
 
 class SiteError(Exception):
     pass
@@ -12,9 +20,26 @@ class SiteNotFoundError(SiteError):
     pass
 
 
+@dataclass(frozen=True, slots=True)
+class SiteCurrentReading:
+    timestamp: datetime | None
+    site_id: str
+    site_type: str
+    consumption_kw: float | None
+    consumption_kwh: float | None
+    voltage_v: float | None
+    current_a: float | None
+    power_factor: float | None
+    temperature_celsius: float | None
+    humidity_percent: float | None
+    null_reasons: list[str]
+    data_quality: DataQuality
+
+
 class SiteService:
-    def __init__(self, *, sites: SiteRepository) -> None:
+    def __init__(self, *, sites: SiteRepository, readings: ReadingRepository) -> None:
         self._sites = sites
+        self._readings = readings
 
     async def list_all(self) -> Sequence[Site]:
         return await self._sites.list_all()
@@ -24,3 +49,41 @@ class SiteService:
         if site is None:
             raise SiteNotFoundError(site_id)
         return site
+
+    async def current(self, site_id: str) -> SiteCurrentReading:
+        site = await self.get_by_id(site_id)
+        derniere = await self._readings.latest_for_site(site_id)
+
+        if derniere is None:
+            return SiteCurrentReading(
+                timestamp=None,
+                site_id=site.site_id,
+                site_type=site.site_type,
+                consumption_kw=None,
+                consumption_kwh=None,
+                voltage_v=None,
+                current_a=None,
+                power_factor=None,
+                temperature_celsius=None,
+                humidity_percent=None,
+                null_reasons=[],
+                data_quality="critical",
+            )
+
+        qualite: DataQuality = (
+            derniere.data_quality if derniere.data_quality in QUALITES_CONNUES else "critical"
+        )
+        return SiteCurrentReading(
+            timestamp=derniere.timestamp,
+            site_id=site.site_id,
+            site_type=site.site_type,
+            consumption_kw=derniere.consumption_kw,
+            consumption_kwh=derniere.consumption_kwh,
+            voltage_v=derniere.voltage_v,
+            current_a=derniere.current_a,
+            power_factor=derniere.power_factor,
+            temperature_celsius=derniere.temperature_celsius,
+            humidity_percent=derniere.humidity_percent,
+            null_reasons=derniere.null_reasons or [],
+            data_quality=qualite,
+        )